Serration in A l-Zn Alloys Containing a Small Amount of Fe

Effect of addition of Fe on the occurrence of serration in Al-12mass%Zn alloys was investigated. Specimens aged at 293K for various periods after quenching from various temperatures (T(Q)), 398K to 823K, were tensile-tested at room temperature. Serration occurred more easily and more remarkably with decreasing T(Q) for as-quenched specimens: in the case that T(Q)=448K serration was observed both for the binary and Fe added alloys, while in the case that T(Q)=573K none of the three alloys showed serration. For the binary alloy serration was observed only when the aging period was short enough, but addition of Fe to the binary alloy prolonged the aging period where serration could be recognized. Aging rate measured by hardness was remarkably retarded with the increase of Fe addition. These results confirm the interpretation that the serration in Al-Zn alloy occurs in the early stage of aging where small GP zones or solute clusters are formed

Effect of Quenching Condition on the Growth of GP Zones in Al-lmass % Ag Alloy

Aging of Al-lmass % Ag alloy at 273K after quenching under various conditions was studied by measurement of electrical resistivity. Scattering of the quasi-equilibrium value of resistivity (p(e)) was not random but closely related to the as-quenched value (p(o)); P(e) increased with increasing p(o)' When the quenching temperature (T(q)) was lower than or equal to 773K, the state at p(e) was controlled substantially by the concentration of quenched vacancy. On the other hand, when T(q)>823K, GP zones formed during quenching played an important role, instead of quenched vacancies, in determining the state

Fluctuation of the Solute Concentration in Al Rich Al-Zn Alloys

Several Al-Zn alloys containing 0.041-4.4 at % Zn were studied by means of measurements of electrical resistivity. The results obtained are as follows : (1) The electrical resistivity increases when the specimen is annealed at temperatures higher than the solvus temperature of the G.P. zones. The increase of the resistivity is due to the formation of fluctuation. (2) The electrical resistivity of the specimen containing fluctuation is dependent upon annealing temperature only and independent of quenching temperature. (3) The fluctuation is formed in very dilute alloys as 0.041 at % Zn at temperatures higher than the solvus temperature of the G.P. zones. (4) The formation energy of vacancy and the migration energy of the Zn atom in the alloys determined by the formation process of fluctuation are in good agreement with those by the formation process of G.P. zones. (5) In spite of the result (4), it seems that the fluctuation is not the same as the small G.P. zones which are observed in the early stage of aging

Effect of Surface and Grain Boundary on the Reversion of Age-Hardened Al-15mass % Zn Alloy

Reversion of the age-hardened Al-15mass % Zn alloy, in which ellipsoidal GP zones were formed, was investigated by hardness test. Ellipsoidal zones were reverted more quickly near the surface and grain boundary than in the interior, as spherical zones did. The results confirm their role as sources for vacancies in reversion

Serration and Reversion Treatment in Al-10mass%Zn Alloy

The relationship between occurrence of serration and reversion heat treatment was studied by tensile test on Al-10mass%Zn alloy. Serration was observed for the furnace cooled specimen, but not for the as-quenched one and the shortly aged one, which result is difficult to be interpreted by the Cottrell theory. The stress amplitude in the serration increased with increasing the time of annealing for reversion up to 10ks and then stayed at a constant value which might be interpreted by the Cottrell theory. The stress amplitude in the serration decreased with increasing the annealing temperature, which is contrary to the prediction by the Cottrell theory. The specimen directly annealed without aging and the one annealed after aging agreed well with each other in the tensile strength and the stress amplitude in the serration, which suggests solute clusters as the cause of serration

Fatigue Strength of Age-Hardened Al-Zn Alloys under Repeated Tensile Loading

Effect of the soft surface layer that was formed on low temperature aging of Al-Zn alloy on fatigue strength was studied under repeated tensile loading. Vickers microhardness test revealed that there existed less hardened region in the vicinity of grain boundary and surface, and that the region extends 50 to 100μm from the surface inward. From the plot of the stress amplitude against the number of cycles to failure, it is concluded that the presence of less hardened surface layer strengthens fatigue resistance of the age hardened Al-Zn alloys containing 8 to 16mass % Zn under the repeated tensile loading

Effect of the Soft Surface Layer on Fatigue Strength of LowTemperature Aged Al-2mass % Cu Alloy

Fatigue strength of Al-Cu alloy was examined by a repeated tensile mode when the specimens were aged and reversion annealed. The specimen quenched from 723K and aged fully around room temperature showed fatigue strength which depended on the existence of soft layer, while the specimen quenched from 723K and aged in the same way showed fatigue strength independent of the existence of the soft layer. Fatigue strength of the specimen, of which the soft surface layer was removed, was the same for either quenching temperature. Fatigue strength became higher when the soft surface layer was thickened with reversion annealing for 600s at 323K after aging. The soft surface layer was thought to increase fatigue strength of the aged Al-Cu alloy as well as Al-Zn alloy

Effect of Heat Treatment on the Serration of Low-Concentrated Al-Zn Alloys

The effect of heat-treatment conditions on occurrence of serration in Al-Zn alloys was investigated. Specimens were aged for various times up to 2.6Ms at 293K or 273K after quenching from various temperatures (T(Q)), 473K to 853K, and tensile-tested at room temperature. Serration occurred more easily according as T(Q) became lower and the aging time became shorter: in the case that T(Q)=473K serration was observed even after aging for 2.6Ms, while in the case that T(Q)=773K serration did not occur irrespective of aging conditions. Serration was also recognized when the specimens were furnace-cooled from 773K to room temperature. These results together with those obtained by the electrical resistometry suggest that the serration in the low'concentrated Al-Zn alloy is caused by the formation of small GP zones whose Guinier radius is less than 1nm or some sort of solute clusters

Low Temperature Age-Hardening of Al-12mass % Zn-0.5mass % Cu Alloy

Effect of addition of a small amount of copper to Al- 12mass % Zn binary alloy on the process of low temperature aging is studied by hardness test and transmission electron microscopy. Age hardening rate after quenching from various temperatures is slowed down by addition of 0.5mass % copper to the binary alloy. The hardness obtained after long aging, however, is increased a little by the addition of copper. The extent of soft surface layer formed by aging in the copper-added alloy is nearly equal to that obtained in the binary alloy